For example, a typical silicon-based solar cell has a configuration including an antireflection film and a light-receiving surface electrode via an n+ layer on an upper surface of a silicon substrate that is a p-type polycrystalline semiconductor and including a back surface electrode (hereinafter simply “electrode” when no distinction is made between these electrodes) via a p+ layer on a lower surface, and electric power generated by receiving light in p-n junction of the semiconductor is extracted through the electrodes. The antireflection film is for the purpose of reducing a surface reflectance while maintaining a sufficient visible light transmittance to increase light reception efficiency and is made up of a thin film of silicon nitride, titanium dioxide, silicon dioxide, etc.
The antireflection film has a high electric resistance value and therefore prevents efficient extraction of electric power generated in the p-n junction of the semiconductor. Therefore, the light-receiving surface electrode of the solar cell is formed with a method called fire-through, for example. In this electrode forming method, for example, after the antireflection film is disposed on the entire surface of the n+ layer, a conductive paste is applied in an appropriate shape onto the antireflection film by using a screen printing method, for example, and is subjected to firing treatment. The conductive paste consists mainly of, for example, silver powder, glass frit (flaky or powdery fragments of glass formed by melting, quenching, and, if needed, crushing glass raw materials), an organic vehicle, and an organic solvent and, since a glass component in the conductive paste breaks the antireflection film in the course of the firing, the ohmic contact is formed between the conductive component in the conductive paste and the n+ layer (see, e.g., Patent Document 1). This electrode forming method simplifies the operation as compared to the case of partially removing the antireflection film to form an electrode in the removed portion and causes no problem of displacement between the removed portion and the electrode forming position.
Various proposals have hitherto been made in such a solar cell light-receiving surface electrode formation for a purpose such as enhancing the fire-through property to improve the ohmic contact and consequently increasing a fill factor (FF) and energy conversion efficiency. For example, the group five elements such as phosphorus, vanadium, and bismuth are added to the conductive paste to promote the oxidation-reduction effect of glass and silver to the antireflection film, improving the fire-through property (see, e.g., Patent Document 1 above). Chloride, bromide, or fluoride is added to the conductive paste to assist the effect of glass and silver breaking the antireflection film with these additives, improving the ohmic contact (see, e.g., Patent Document 2). The glass is borosilicate glass, for example.
It is proposed to include 0.5 to 5 parts by weight of silver phosphate per 100 parts by weight of silver powder into the conductive paste so as to assist the effect of breaking the antireflection film and ensure the ohmic contact (see, e.g., Patent Document 3). It is described that when glass containing zinc oxide as a major component without containing lead is used for forming a paste containing silver, gold, and antimony, the breakage of conjunction does not occur because of the absence of penetration of an electrode, thereby ensuring a low contact resistance (see, e.g., Patent Document 4). It is also proposed for a silver-containing paste containing 85 to 99 (wt %) silver and 1 to 15 (wt %) glass that the glass has a composition containing 15 to 75 (mol %) PbO and 5 to 50 (mol %) SiO2 and not containing B2O3 (see, e.g., Patent Document 5). This silver-containing paste is used for the solar cell electrode formation and the ohmic contact is considered to be improved by using the glass having the composition described above. The glass can contain 0.1 to 8.0 (mol %) P2O5 or 0.1 to 10.0 (mol %) Sb2O5 and can further contain 0.1 to 15.0 (mol %) alkali metal oxide (Na2O, K2O, Li—2O).